Diagnosing shortwave cryosphere radiative effect and its 21st century evolution in CESM

Abstract

We incorporate a new diagnostic called the cryosphere radiative effect (CrRE), the instantaneous influence of surface snow and sea ice on the top-of-model solar energy budget, into two released versions of the Community Earth System Model (CESM1 and CCSM4). CrRE offers a more climatically relevant metric of the cryospheric state than snow and sea ice extent and is influenced by factors such as the seasonal cycle of insolation, cloud masking, and vegetation cover. We evaluate CrRE during the late 20th century and over the 21st century, specifically diagnosing the nature of CrRE contributions from terrestrial and marine sources. The radiative influence of ice sheets and glaciers is not considered, but snow on top of them is accounted for. Present-day global CrRE in both models is -3.8 W m2, with a boreal component (-4.2 to -4.6 W m-2) that compares well with observationally derived estimates (-3.9 to -4.6 W m-2). Similar present-day CrRE in the two model versions results from compensating differences in cloud masking and sea ice extent. Over the 21st century, radiative forcing in the Representative Concentration Pathway (RCP) 8.5 scenario causes reduced boreal sea ice cover, austral sea ice cover, and boreal snow cover, which all contribute roughly equally to enhancing global absorbed shortwave radiation by 1.4–1.8 Wm-2. Twenty-first century RCP8.5 global cryospheric albedo feedback are +0.41 and +0.45 W/m-2/K, indicating that the two models exhibit similar temperature-normalized CrRE change.